Despite continued advances in magnetic resonance (MR) instrumentation, imperfections in the magnetic field evolution still hamper numerous MR procedures. Field perturbations are caused by a range of mechanisms, including eddy currents, limited gradient bandwidth, and heating effects. Often such errors can be addressed by means of signal processing. However, to do so these errors need to be accurately known. Reproducible field perturbations can be determined approximately by preparatory measurements. Alternatively, it has been proposed to monitor the relevant field evolution directly during each actual scan, using appropriately designed magnetic field probes.
Published European Patent Application EP 1 582 886 A1 discloses a method of MR imaging wherein additional data are acquired, during execution of a MR sequence, from at least one monitoring magnetic field probe positioned in the vicinity of and surrounding an object of interest. The magnetic field probes exploit the presence of a strong magnetic field needed for the MR measurements; accordingly, they are based upon magnetic resonance in a small sample volume of an MR active substance. Upon MR excitation of the object of interest, there is a concomitant MR excitation of the substance within the probe, the resonance frequency of which is proportional to the magnitude of the local magnetic field.
As discussed in EP 1 582 886 A1, magnetic resonance in the probe sample can be based on nuclear magnetic resonance (NMR), in which case the probe sample shall contain a NMR active nuclear species such as 1H, 13C, 17O, 19F or 31P. Magnetic resonance imaging (MRI) is mostly based on 1H. As is well known, the choice of the nucleus for probing is subject to several considerations, and, based on the chosen nucleus, the choice of the substance used as the sample in a magnetic field probe is also subject to several considerations. According to EP 1 582 886 A1, normal water (1H2O) is a preferred probe substance based on 1H whereas suitable substances based on 19F include hexafluorobenzene and trifluoromethlysulfonate.
A key challenge in designing magnetic field probes based on magnetic resonance (henceforth also called “MR-based magnetic field probes”) is obtaining strong and long-lived signals from probe samples that are small enough to avoid dephasing by externally applied gradients. Preferably, the probe samples should have a volume of less than 1 μl. For extracting strong signals from such a small sample it is essential to mount it tightly in a receiver coil. However, nearby material interfaces tend to induce magnetic field variations in the probe sample, thus limiting its signal lifetime. For example, when using water as the probe substance by placing a small water droplet in a thin glass capillary surrounded just by air and a tightly wound copper solenoid, impractically short signal lifetimes of less than 10 ms are achieved.
An improved MR-based magnetic field probe has been described in De Zanche N, Barmet C, Meier D, Pruessmann K. NMR probes for magnetic field monitoring during MRI. Proceedings 13th Scientific Meeting, International Society for Magnetic Resonance in Medicine; 2005, May, p 791. High signal-to-noise signal detection was performed inductively by means of solenoidal microcoils connected to low-noise preamplifiers. Cross-talk was limited primarily by reducing unwanted inductive pickup. All signal paths or inductors that could receive NMR signal because of their geometrical arrangement were either shielded or routed to minimize flux linkage with nuclear spins within the imaging volume. The circuit board containing components for detuning, matching and preamplification was consequently shielded and its connection to the solenoid containing the sample was made using a coaxial cable. Since such a probe is to be excited by an external magnetic field and thus cannot be shielded, particular attention was devoted to minimize the sensitivity of the solenoid to spins outside of its interior. To reduce field inhomogeneities within the sample, susceptibility matching techniques were employed.
In a specific embodiment of the magnetic field probe disclosed by De Zanche, loc. cit., a small droplet of water was injected inside a precision 2.2 mm inner diameter pyrex capillary previously filled with a perfluorinated hydrocarbon (FC-77 Fluorinert®; 3M, USA, henceforth simply called “FC77”). FC77 was also used to displace air within and around the solenoid, enclosing the complete probe within a 2 cm diameter cylinder. Due to the residual susceptibility mismatch between FC77 and copper, the solenoid's diameter was chosen to be 0.5 mm larger than the capillary's 2.5 mm outer diameter.
A disadvantage of the above described magnetic field probe is due to the fact that the magnetic susceptibilities of copper (−9.65 ppm) and FC-77 (approximately −8 ppm) are by no means identical. This residual susceptibility mismatch causes an undesirable shortening of the probe's resonance signal. A further disadvantage is caused by the fact that FC-77 is a liquid at room temperature, which is impractical for the purpose of enclosing the complete probe.
Planar microcoil-based microfluidic NMR probes have been described in Massin, C. et al., Journal of Magnetic Resonance; 164 (2003) pp. 242-255, the contents of which are incorporated herein by reference. These probes comprise electroplated planar microcoils integrated on a glass substrate with etched microfluidic channels. The main factor limiting sensitivity for high-resolution applications has been reported by Massin, loc. cit. as being probe-induced static magnetic field distortions; these are mainly caused by differences in bulk magnetic susceptibility of materials composing the probe.
U.S. Pat. No. 3,091,732 discloses a gyromagnetic resonance probe having transmitter and receiver coils embedded in a magnetic susceptibility matched material comprising by weight about 3 parts of paramagnetic manganese dioxide to 100 parts by weight of a diamagnetic epoxy resin.